This invention relates to methods for the regeneration of a denitration catalyst. More particularly, it relates to methods for the regeneration of a denitration catalyst which makes it possible to regenerate a denitration catalyst having reduced denitration powder and considered to be hard to regenerate, and thereby utilize it again effectively.
Recently, in order to remove nitrogen oxides (hereinafter referred to as NOx) produced in boilers and various combustion furnaces for the purpose of preventing air pollution, a catalytic ammonia reduction process wherein ammonia is used as a reducing agent and nitrogen oxides are decomposed to nitrogen and water by contact with a catalyst is being widely employed. Most of the NOx removal catalysts currently used for practical purposes are honeycomb-shaped catalysts which have through-holes of square cross section in order to prevent clogging with dust present in exhaust gas and increase the gas contact area.
With respect to catalyst components, titanium oxide is highly suitable for use as a principal component, and vanadium, tungsten and the like are commonly used as active components. Thus, TiO2-WO3 or TiO2-MoO3 binary catalysts and TiO2-V2O5-WO3 or TiO2-V2O5-MoO3 ternary catalysts are being popularly used. The catalytic power of these denitration catalysts tends to be gradually reduced with service time, and the cause for the reduction in catalytic power varies according to the type of the fuel used in the source of exhaust gas (e.g., boiler).
For example, in the case of exhaust gas from an oil-fired boiler, sodium contained in the dust present in exhaust gas is chiefly deposited on the catalyst and causes a reduction in catalytic power. In the case of exhaust gas from a coal-fired boiler, calcium contained in the dust present in exhaust gas is chiefly deposited on the catalyst surfaces and reacts with sulfuric anhydride present in the exhaust gas to form calcium sulfate. This calcium sulfate covers the catalyst surfaces and hinders NO and NH3 gases from diffusing into the interior of the catalyst, resulting in reduced catalytic power.
It has conventionally been known that catalysts having reduced catalytic power attributable to these causes of deterioration can be effectively regenerated by cleaning them with water or an aqueous solution of hydrochloric acid.
In the course of experiments on the regeneration of catalysts having been used for exhaust gas from coal-fired boilers, the present inventors have recognized that the conventional cleaning method using water or an aqueous solution of hydrochloric acid exhibits is scarcely effective in regenerating the catalytic power of some catalysts. Upon examination of the cause therefor, it has been found that a high concentration of arsenic compound(As2O5) is present on the surfaces of the catalysts for which cleaning with water or an aqueous solution of hydrochloric acid fails to exhibit a regenerative effect.
Generally, when a denitration catalyst is applied to exhaust gas produced by the combustion of a gaseous fuel, little reduction in catalytic power is observed.
However, for catalysts used in exhaust gas from coal-fired boilers in which coal of poor quality tends to be increasingly used in recent years, a marked reduction in catalytic power is observed in some cases. Examination of these deteriorated catalysts has revealed that a high concentration of arsenic is present on the catalyst surfaces as described above, and the conventional cleaning method using water or an aqueous solution of hydrochloric acid exhibits little regenerative effect on them. Moreover, in order to clarify the cause for the deposition of arsenic on the surfaces of a catalyst used for a coal-fired boiler, an investigation was made on the fuel used in the source of exhaust gas. As a result, it has been found that a high concentration of arsenic compounds are present in such coal. These arsenic compounds are converted into diarsenic trioxide (As2O3), which is carried by combustion gas and becomes adsorbed on the catalyst. Then, this diarsenic trioxide is oxidized on the catalyst according to the following reaction formula (1) and fixed to the catalyst in the form of stable diarsenic pentoxide (As2O5).
As2O3+O2xe2x86x92AS2O5xe2x80x83xe2x80x83(1) 
For this reason, there has been a problem in that, when the substances responsible for the deterioration of the catalyst are arsenic compounds deposited on the catalyst surfaces, the conventional cleaning method using water or an aqueous solution of hydrochloric acid exhibits little regenerative effect on the catalyst.
In view of the above-described problem, the present inventors made intensive investigations in order to develop a method for the regeneration of a denitration catalyst which not only can regenerate a denitration catalyst having reduced catalytic power as a result of its long-time use, while avoiding the conventionally known reduction in catalytic power due to the deposition of sodium or calcium, but also can regenerate a denitration catalyst that could not be effectively regenerated by cleaning with water or an aqueous solution of hydrochloric acid because of the presence of arsenic on the catalyst surfaces.
As a result, the present inventors have now found that the above-described problem can be solved by treating a spent denitration catalyst according to a method which comprises an alkali treatment step for removing the arsenic compounds deposited on the catalyst surfaces, and a subsequent activation treatment step.
Moreover, the present inventors also have found that the above-described problem can be solved by cleaning a spent denitration catalyst with an aqueous solution of sulfuric acid or ammonia to convert the arsenic compounds deposited on the catalyst surfaces into water-soluble compounds and thereby remove them from the catalyst surfaces.
The present invention has been completed from this point of view.
According to a first embodiment of the present invention, there is provided a method for the regeneration of a denitration catalyst which comprises cleaning a denitration catalyst having reduced denitration power with an aqueous alkaline solution to remove the substances deposited thereon, and subjecting the catalyst to an activation treatment with an aqueous acid solution. In a preferred embodiment, the aforesaid aqueous alkaline solution is an aqueous solution of NaOH, KOH, Na2CO3, NaHCO3 or K2CO3 and the aforesaid aqueous acid solution is an aqueous solution of HCl, HNO3, HF or H2SO4.
According to a second embodiment of the present invention, there is provided a method for the regeneration of a denitration catalyst which comprises cleaning a denitration catalyst having reduced denitration power with a cleaning fluid comprising an aqueous solution containing sulfuric acid or ammonia at a concentration of 0.05 to 20% by weight and maintained at a temperature of 10 to 90xc2x0 C. In this method, the hardly soluble arsenic compounds deposited on the catalyst surfaces can be more effectively removed by maintaining the temperature of the cleaning fluid in the range of 20 to 80xc2x0 C.
According to a third embodiment of the present invention, there is provided a method for the regeneration of a denitration catalyst which comprises cleaning a denitration catalyst having reduced denitration power under any of the conditions described above, and impregnating the denitration catalyst with a catalytically active component so as to support it on the catalyst. In this method, the catalytically active component with which the catalyst is impregnated comprises, for example, vanadium or tungsten that is liable to be dissolved out.
Conventionally, catalysts having arsenic compounds deposited thereon have been incapable of regeneration and hence disposed of. However, the regeneration methods of the present invention make it possible to regenerate such catalysts and utilize them effectively again as denitration catalysts.